Powder metallurgy techniques have been used to formulate refractory metal composite materials with both high hardness and high impact strength. U.S. Pat. No. 4,024,902 describes a composite material made from cemented carbide particles containing tungsten carbide and cobalt, the cemented carbide particles being placed in a mold and infiltrated with molten steel alloy. The tungsten carbide and cobalt dissolved in the steel alloy and then precipitate from the alloy as the article is cooled. The resultant composite article contains particles of tungsten carbide surrounded by successive shells containing tungsten (from the tungsten carbide), carbon (from the tungsten carbide), cobalt, and steel, each of these shells having lower hardness than the tungsten carbide particles. The remainder of the article is occupied by the steel alloy. The hardest material in such a composite is tungsten carbide, and the softest material in such a composite is steel alloy. U.S. Pat. No. 4,140,170 describes an improvement in the molding process of U.S. Pat. No. 4,024,902. According to the method of the latter patent, sintered tungsten carbide is ground up and mixed with iron powder. The powder mixture is then packed in a mold and heated to form a composite material. The methods of these patents employ liquid phase reactions which are not suitable for the precision replication of a molded shape, due to dimensional changes which occur as the materials within the composite chemically combine with one another.
U.S. Pat. No. 3,258,817 describes a composite material made by placing spheroidal, refractory, hard metal particles in a mold, infiltrating the particles with a molten binder metal having a melting point between 816.degree. C. and 1649.degree. C., and cooling the infiltrated article. The refractory particles partially dissolve in the binder metal during infiltration, then precipitate from the binder during cooling of the article. The process conditions are said to be preferably controlled so as to cause an "intergrowth" of the refractory granules and formation of a continuous hard metal phase. Such a composite material would have low impact resistance due to the interconnection or intergrowth of refractory granules, since this would provide an efficient pathway for crack propagation through the material. Also, the method of this patent might be unsuitable for the precision replication of a molded shape due to the use of liquid phase reactions.
U.S. Pat. Nos. 3,823,002 and 3,929,476 describe precision shaped articles, such as electrical discharge machining electrodes, made by molding in a flexible mold a plastic mixture of multimodal refractory powders and a thermoplastic binder to form a green molded article of predetermined shape and dimensions, heating the green molded article to remove the binder and consolidate the refractory powders into an interconnected skeletal structure, and infiltrating the resulting skeletal structure with a molten infiltrant which is a low melting point metal or alloy.
U.K. published Patent Application No. 2,005,728 A describes a molded, non-refractory metal article made by molding in a flexible mold a plastic mixture of non-refractory, spherical metal powders and heat-fugitive binder comprising thermoplastic material to form a green article of predetermined shape and dimensions, heating the green article to remove the binder and consolidate the non-refractory spherical powders in the form of a porous, monolithic skeleton of necked particles of non-refractory metal, infiltration the skeleton with a molten metal having a melting point that is at least 25.degree. C. less than the melting point of the lowest melting of the spherical, non-refractory particles, and cooling the metal infiltrated skeleton thereby forming a homogeneous, void-free non-refractory metal article of two intermeshed metal matrices. The molded skeleton may be made of particles of Fe, Co, Ni, or their alloys and the infiltrant metal may be Cu, Ag, or Sn.